Merging technique for OTDR traces captured by using different settings

ABSTRACT

An Optical Time Domain Reflectometer (OTDR) tests an optical fiber by generating, transmitting, and receiving light signals from an optical fiber. The OTDR generates light signals having different characteristics and stitches these light signals into an OTDR trace. Backscatter and properties such as dynamic range effect the quality of the OTDR trace.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromU.S. Provisional Patent Application No. 62/074,883, filed Nov. 4, 2014in the United States Patent and Trademark Office, and as a U.S. nationalstage filing of International Application No. PCT/US2015/058997 havingan international filing date of Nov. 4, 2015, the disclosures of both ofwhich are incorporated by reference herein in their entireties.

TECHNICAL FIELD

Exemplary embodiments relate to an Optical Time Domain Reflectometer(OTDR), and more particularly to merging OTDR traces generated by anOTDR using different settings respectively.

BACKGROUND ART

An OTDR is an instrument used to detect and characterize features in anoptical fiber. These features may include spontaneous increase of signallevel due to reflection at a small air gap between parts of a connector,or a drop of signal level caused by a splice loss or a power splitter.In some situations, reflectance or optical loss could represent adefective fiber.

An OTDR analyzes optical signals through an optical fiber and may outputsuch signals as OTDR traces. OTDR traces are generally captured in thepresence of noise making it difficult to detect or reliably characterizesignal characteristics having a magnitude less than that of the noise.

In addition to noise, dynamic range and dead-zone of OTDR traces arerelevant to OTDR instrumentation. Dynamic range is a backscatter levelat the entrance of a fiber compared to noise level such that a betterdynamic range indicates a smaller noise level. Dead-zone is a section ofan OTDR trace immediately following a spontaneous increase of signallevel due to reflection, such that a trace may appear functionallysaturated, before the power of received light falls within some level,such as 1.5 dB for reflective event and 0.5 dB for non-reflective event.

Present techniques for overcoming such noise and inability to interpretsignal characteristics, such as within a dead-zone of an OTDR trace,have disadvantageous. Increasing the power of a light pulse may increasethe complexity and price of an instrument, as well as may cause safetyissues. Averaging repeated measurement results may increase test timeand thereby decrease productivity. Increasing a duration of a lightpulse may cause an OTDR trace to exhibit at least longer dead-zonesthereby preventing any measurement during a greater portion of thetrace. Applying a filter to an OTDR trace may have similar effect asincreasing a duration of a light pulse, and the effect of the filter maydiminish as length of the filter increases. Accordingly, a technique forOTDR traces is needed which may overcome these disadvantages.

Solution to Problem

In accordance with an aspect of the present invention(s), there areseveral modifications to OTDR systems that can provide improvedmeasurement accuracy. This includes capturing a series of OTDR traces,that use different light pulse-widths optimized to capture differentcharacteristics of the same test network. These traces are then stitchedtogether to produce a composite or merged trace that uses the optimumelements from each individual trace. This composite trace providesshorter dead-zones with higher noise immunity than a single trace usinga single pulse-width.

It is an aspect of the exemplary embodiments to provide an OTDR capableof dealing with events and short pulse-width signals without sufficientdynamic range. For example, for a fast falling of a short pulse-widthtrace, an OTDR may be configured to determine a point where a shorterpulse-width trace and a longer pulse-width trace intercept. If an OTDRtrace distance to the point of intersection of the two traces exceeds adistance of a dead-zone of the longer pulse-width trace and there is nosignificant signal level rise within this distance, then the OTDR willadd data from at least the shorter pulse-width trace, or its filteredversion, to an OTDR trace, even when the shorter pulse-width changesrapidly. The OTDR trace may continue to add data from the shorterpulse-width trace until the two traces reach the same level, at whichpoint the longer pulse-width trace and shorter traces may be stitchedtogether, or other optical characteristics are detected.

Advantageous Effects of Invention

Advantages and benefits of the above-described exemplary embodimentsinclude, but are not limited to merged traces having simultaneouslylower backscatter noise levels and a shorter dead-zone, overall, ascompared to a trace captured with a pulse-width longer than the shortestpulse-width and shorter than the longest pulse-width used before a mergeor stitching process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an OTDR system including input and output sectionsaccording to exemplary embodiments;

FIG. 2 illustrates a selector section according to exemplaryembodiments;

FIGS. 3A and 3B illustrates OTDR traces according to exemplaryembodiments;

FIG. 4 illustrates a flowchart according to exemplary embodiments.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings, wherein likereference numerals refer to like elements throughout. However, knownfunctions associated with the exemplary embodiments or detaileddescriptions on the configuration and other matters which wouldunnecessarily obscure the present disclosure will be omitted.

FIG. 1 is a view of an OTDR system 1000 including a light generationsection 101, a light I/O 102, an exit path 103, a return path 104, adetection section 105, a selector 106, a processor 107, a display 108and a memory or storage 109.

The memory or storage 109 may store executable instructions that, whenexecuted by the processor 107, cause the processor to perform algorithmsaccording to exemplary embodiments.

According to exemplary embodiments, the processor 107 of the OTDR system1000 algorithms control the light generation section 101 to generatelight pulses having different durations, pulse-width, to be sent to afiber through exit path 103 of the light I/O 102 and to be reflectedfrom the fiber through the return path 104 of the light I/O 102. Theprocessor 107 may also be configured to control the light generationsection 101 to generate light pulses not only having differentpulse-width respectively, but also having different intensity andrepetitions of previously transmitted light signals according toalgorithms.

The detection section 105 receives the light signals, having at leastrespectively different pulse-width, from the return path 104 of thelight I/O 102 and converts the light signals into electrical signals,such as by a photodetector according to exemplary embodiments.

The optical signals, having at least different pulse-width, may befurther processed with respective gains, such as a low-level gain toaccommodate high-amplitude signal at a near end, close to the OTDR, anda high-level gain to accommodate a low-amplitude signal for a far end,away from the OTDR, along a fiber. As discussed further below, theoptical signals may be stitched into a single OTDR trace in a horizontaldirection of the trace, where the horizontal direction of the trace mayrepresent distance from the OTDR.

Various methods for processing light signals according to exemplaryembodiments include, increasing power of light pulse, repeating andaveraging measurements, increasing duration of a light pulse andfiltering captured traces, at least to reduce noise levels.

The selector 106 receives the electrical signals corresponding to thelight signals, having at least respectively different pulse width, andmay be controlled by the processor 107 to select various ones of theelectrical signals and various portions of the electrical signals forbeing output to an OTDR trace by the display 108, as further discussedwith respect to the flowchart of FIG. 4.

The processor 107 further stores such data in a memory 109 and processesthe stored data to provide outputs to at least the display 108.

FIG. 2 is a view of a selector section 2000 including the selector 106having an input 210 and an output 220. The input 210 and the output 220may each represent more than one signal path according to exemplaryembodiments. A selection unit 205 of the selector 106 operates inconjunction with the processor 107 or may autonomously operate todetermine signals and portions of signals to be displayed by the display108. The selector 106 further includes receivers 201-205 each configuredto receive and buffer signals corresponding to at least respectivepulse-width denoted by “L”.

FIG. 3A illustrates a graph 3000 a including various OTDR traces 301a-303 a each representing light signals having different pulse-width.For example, trace 301 a may represent a light signal having a 5 nspulse-width; trace 302 a may represent a light signal having a 30 nspulse-width, and trace 303 a may represent a light signal having a 300ns pulse-width.

To reduce scaling errors of the traces, backscatter levels of at leasttwo traces obtained by using different pulse-width may be normalized bymultiplying each trace by a product of pulse-width, number of averagesand gain of another trace, according to exemplary embodiments. Thesecoefficients could be divided by a maximum common denominator before thenormalization to avoid processing overflow. Although at least two tracesmay be normalized as described above, more than two traces may also benormalized similarly by multiplying each trace by a product ofpulse-width, number of averages and gain of another trace, according toexemplary embodiments.

The trace 302 a ends its respective dead-zone and intersects the trace301 a at stitching point 310. The trace 303 a ends its respectivedead-zone and intersects the trace 302 a at stitching point 311.

As illustrated in FIG. 3A, the trace 301 a experiences the shortestdead-zone, and the trace 303 a experiences the longest dead-zone.Further, the trace 301 a experiences the greatest amount of noise afterits respective dead-zone, and the trace 303 a experiences the leastamount of noise after its respective dead-zone. The shorter pulse-widthtrace 301 a, although noisier than the longer pulse width trace 303 a,exhibits optical characteristics at a shorter fiber distance than thelonger pulse-width trace 303 a at least because of the shorter dead-zoneof the shorter pulse-width trace 301 a; further, the characteristics atthe shorter fiber distance exhibited by the shorter pulse-width trace301 a are not exhibited by the longer pulse-width trace 303 a because ofits respective dead-zone.

FIG. 3B illustrates a graph 3000 b having trace portions 301 b-303 bstitched together from the traces 301 a-303 a of graph 3000 a. Forexample, as trace 301 a experiences a shortest dead-zone by representingat least a shortest pulse-width, the portion 301 b of the trace 301 a isselected as a portion to be displayed by the graph 3000 b.

The trace 302 b experiences less noise than the trace 301 a; however,the trace 302 b has a longer dead-zone. The dead-zone of the trace 302 bends at stitching point 310 and therefore, the trace 302 b may beprioritized over the noisier trace 301 a. A trace portion trace 302 b ofthe trace 302 a is stitched to the noisier trace portion 301 b.

Although trace portion trace 301 b is noisier than trace portion 302 b,the trace portion 301 b provides non-dead-zone data at a shorterdistance than could be reliably represented the longer pulse representedby the trace 301 b.

Further, the stitching point 311 represents a point where the dead-zoneof the trace 303 a has ended and is prioritized over the noisier trace302 a, and therefore, the trace portion 303 b may be stitched atstitching point 311 thereby providing a less noisy signal at greaterdistances from the OTDR.

The stitching may be progressive rather than sudden. For example, areasaround the stitching point 310 and stitching point 311 may be ratios ofthe signals about the respective points. The ratio may be different ateach stitching point.

Further, the traces 301 a-303 a also have different gains appliedthereto respectively in addition to representing different pulse-widthsignals and therefore may reduce scaling of an OTDR trace.

Meanwhile, if the data of the stitched OTDR trace comes from either theshorter pulse-width signal or the longer pulse-width signal, there maybe one unused trace. According to exemplary embodiments, the longerpulse-width signal may be replaced by a combined trace from the twotraces according to the following formula:L′(n)=a*L(n)+b*S′(n)  (1)

L(n) denotes a longer pulse-width signal, S′(n) denotes a moving averagefiltered version of a shorter pulse-width signal S(n); “a” and “b” aretwo parameters to be chosen such that a ratio of total noise of L′ (n)to combined minimizes a backscatter noise level. It is noted that thepulse-width of S(n) is Ps, the shorter pulse-width, and in order toobtain S′(n) in view of the longer pulse-width P1 as L(n), the optimummoving average filter length to compute S′(n) from S(n) should be P1-Ps.

FIG. 4 illustrates flowchart 4000 of an algorithm performed according toexemplary embodiments. At S400 an OTDR receives signals representing atleast different pulse-width.

At S401, the OTDR determines that the signals rise spontaneously andamplitude of a short pulse-width trace is greater than amplitude of alonger pulse-width trace.

At S402, the OTDR adds data corresponding to the short pulse-widthsignal even though this signal may be noisy. A filtered version of theshort pulse-width signal may be added.

At S403, the OTDR compares the amplitude of the short pulse-width signalto that of a longer pulse-width signal.

At S404, the OTDR has determined that the amplitude of the shortpulse-width signal is greater than that of the longer pulse-widthsignal, and therefore, the longer pulse-width signal remains in adead-zone. Processing returns to S403.

This process may continue until the short pulse-width trace has a numberof points having negative value exceeding a defined threshold such thatthe short pulse trace no longer has a sufficient dynamic range. Fromthis point, the short pulse-width trace may only be selected in case ofa large spontaneous rise of signal level, according to exemplaryembodiments.

However, at S405, the OTDR has determined that the amplitude of theshort pulse-width signal is less than or equal to that of the longerpulse-width signal, and therefore, the longer pulse-width signal isoutside of its dead-zone and the OTDR stitches the longer-pulse-widthsignal or a filtered version of the longer pulse-width signal onto anOTDR trace. The above formula may represent multiple ratios for any of afirst light signal and subsequent light signals having respectivelydifferent pulse-width.

Although exemplary embodiments of the disclosure have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these exemplary embodiments without departingfrom the principles and spirit of the exemplary embodiments, the scopeof which is defined in the claims and their equivalents.

REFERENCE SIGNS LIST

OTDR 100

light generation section 101

light I/O 102

exit path 103

return path 104

detection section 105

selector 106

processor 107

display 108

memory 109

receivers 301-305

selection unit 205

input 210

output 220

traces 301 a-303

traces 301 b-303 b

stitching point 310

stitching point 311

OTDR system 1000

selector section 2000

graph 3000 a

graph 3000 b

flowchart 4000

What is claimed is:
 1. An Optical Time Domain Reflectometer (OTDR)comprising: a processor; and a non-transitory, tangible memory storingexecutable instructions that, when executed by the processor, cause theprocessor to perform: generating a first light signal comprising a lightpulse-width having a first duration; generating a second light signalcomprising a light pulse-width having a second duration longer than thefirst duration; generating a third light signal comprising a lightpulse-width having a third duration longer than the second duration;transmitting the first light signal, the second light signal and thethird light signal to an optical fiber; receiving reflections of thefirst light signal, the second light signal and the third light signalfrom the optical fiber; converting the reflections into a firstelectrical signal, corresponding to the first light signal, a secondelectrical signal, corresponding to the second light signal, and a thirdelectrical signal, corresponding to the third light signal; generating afirst OTDR trace corresponding to the first electrical signal, a secondOTDR trace corresponding to the second electrical signal, and a thirdOTDR trace corresponding to the third electrical signal; stitching aportion of the first OTDR trace to a portion of the second OTDR trace atan intersection of the second electrical signal with the firstelectrical signal, whereby a ratio of total noise to be combinedminimizes a backscatter noise level; and stitching the portion of thesecond OTDR trace to a portion of the third OTDR trace at anintersection of the third electrical signal with the second electricalsignal, whereby a ratio of total noise to be combined minimizes abackscatter noise level.
 2. The OTDR of claim 1, wherein stitching theportion of the first OTDR trace to the portion of the second OTDR tracefurther comprises mixing a first ratio of the first OTDR trace andsecond OTDR trace into points of an output OTDR trace proximate thefirst stitching point, and wherein stitching the portion of the secondOTDR trace to the portion of the third OTDR trace further comprisesmixing a second ratio of the second OTDR trace and the third OTDR traceinto points of the output OTDR trace proximate the second stitchingpoint.
 3. The OTDR of claim 2, wherein the first ratio and the secondratio satisfy the following:L′(n)=a*L(n)+b*S′(n) L(n) denotes the second light signal for the firstratio; L(n) denotes the third light signal for the second ratio; “a” and“b” denote two parameters to be chosen such that a ratio of total noiseof L′(n) to be combined minimizes a backscatter noise level; S′(n) is amoving average filter with a length P1-Ps, where P1 represents thesecond duration and Ps represents the first duration for the firstratio; and S′(n) is a moving average filter with a length P1-Ps, whereP1 represents the third duration and Ps represents the second durationfor the second ratio.
 4. The OTDR of claim 1, wherein the stitchingfurther comprises: applying a first gain to the first electrical signal;applying a second gain, different than the first gain, to the secondelectrical signal; applying a third gain, different than the first gainand second gain, to the third electrical signal; re-testing the firstlight signal on the optical fiber; re-testing the second light signal onthe optical fiber; re-testing the third light signal on the opticalfiber; averaging a result of re-testing the first light signal;averaging a result of re-testing the second light signal; averaging aresult of re-testing the third light signal; multiplying the first OTDRtrace by a product of the light pulse-width having the first durationand the light pulse width having the second duration, a number ofaverages and the second gain; multiplying the second OTDR trace by aproduct of the light pulse-width having the first duration and the lightpulse width having the second duration, the number of averages and thefirst gain; and multiplying the third OTDR trace by a product of thelight pulse-width having the second duration and the light pulse widthhaving the third duration, the number of averages and one of the firstor second gain.
 5. A method of using an Optical Time DomainReflectometer (OTDR) comprising: generating a first light signalcomprising a light pulse-width having a first duration; generating asecond light signal comprising a light pulse-width having a secondduration longer than the first duration; generating a third light signalcomprising a light pulse-width having a third duration longer than thesecond duration; transmitting the first light signal, the second lightsignal and the third light signal to an optical fiber; receivingreflections of the first light signal, the second light signal and thethird light signal from the optical fiber; converting the reflectionsinto a first electrical signal, corresponding to the first light signal,a second electrical signal, corresponding to the second light signal,and a third electrical signal, corresponding to the third light signal;generating a first OTDR trace corresponding to the first electricalsignal, a second OTDR trace corresponding to the second electricalsignal, and a third OTDR trace corresponding to the third electricalsignal; stitching a portion of the first OTDR trace to a portion of thesecond OTDR trace at an intersection of the second electrical signalwith the first electrical signal, whereby a ratio of total noise to becombined minimizes a backscatter noise level; stitching the portion ofthe second OTDR trace to a portion of the third OTDR trace at anintersection of the third electrical signal with the second electricalsignal, whereby a ratio of total noise to be combined minimizes abackscatter noise level.
 6. The method of claim 5, wherein stitching theportion of the first OTDR trace to the portion of the second OTDR tracefurther comprises mixing a first ratio of the first OTDR trace and thesecond OTDR trace into points of an output OTDR trace proximate thefirst stitching point, and wherein stitching the portion of the secondOTDR trace to the portion of the third OTDR trace further comprisesmixing a second ratio of the second OTDR trace and the third OTDR traceinto points of the output OTDR trace proximate the second stitchingpoint.
 7. The method of claim 6, wherein the first ratio and the secondratio satisfy the following:L′(n)=a*L(n)+b*S′(n) L(n) denotes the second light signal for the firstratio; L(n) denotes the third light signal for the second ratio; “a” and“b” denote two parameters to be chosen such that a ratio of total noiseof L′(n) to be combined minimizes a backscatter noise level; S′(n) is amoving average filter with a length P1-Ps, where P1 represents thesecond duration and Ps represents the first duration for the firstratio; and S′(n) is a moving average filter with a length P1-Ps, whereP1 represents the third duration and Ps represents the second durationfor the second ratio.
 8. The method of claim 5, wherein the stitchingfurther comprises: applying a first gain to the first electrical signal;applying a second gain, different than the first gain, to the secondelectrical signal; applying a third gain, different than the first gainand the second gain, to the third electrical signal; re-testing thefirst light signal on the optical fiber; re-testing the second lightsignal on the optical fiber; re-testing the third light signal on theoptical fiber; averaging results of re-testing the first light signal;averaging results of re-testing the second light signal; averagingresults of re-testing the third light signal; multiplying the first OTDRtrace by a product of the light pulse-width having the first durationand the light pulse width having the second duration, a number ofaverages and the second gain; multiplying the second OTDR trace by aproduct of the light pulse-width having the first duration and the lightpulse width having the second duration, the number of averages and thefirst gain; and multiplying the third OTDR trace by a product of thelight pulse-width having the second duration and the light pulse widthhaving the third duration, the number of averages and one of the firstor second gain.
 9. A method of generating a merged trace along anoptical fiber with an Optical Time Domain Reflectometer (OTDR), themethod comprising: generating a first light signal with a lightgeneration section of the OTDR, the first light signal having a firstpulse width; generating a second light signal with the light generationsection of the OTDR, the second light signal having a second pulse widthgreater than the first pulse width; generating a third light signal withthe light generation section of the OTDR, the third light signal havinga third pulse width greater than the second pulse width; transmittingthe first light signal, the second light signal and the third lightsignal to the optical fiber from a light Input/Output of the OTDR;receiving reflections of the first light signal, the second light signaland the third light signal from the optical fiber with a detectionsection of the OTDR; converting the reflections into a first electricalsignal corresponding to the first light signal, a second electricalsignal corresponding to the second light signal, and a third electricalsignal corresponding to the third light signal with the detectionsection of the OTDR; and generating a merged OTDR trace by stitching aportion of the first electrical signal to a portion of the secondelectrical signal at a first stitching point with a selector of theOTDR, stitching the portion of the second electrical signal to a portionof the third electrical signal at a second stitching point with theselector of the OTDR, and outputting the stitched portions to a displayof the OTDR; wherein the first stitching point is defined at anintersection of the second electrical signal with the first electricalsignal such that a ratio of total noise to be combined minimizes abackscatter noise level and the second stitching point is defined at anintersection of the third electrical signal with the second electricalsignal such that a ratio of total noise to be combined minimizes abackscatter noise level.
 10. The method of claim 9, wherein stitchingthe portion of the first electrical signal to the portion of the secondelectrical signal further comprises mixing a first ratio of the firstelectrical signal and the second electrical signal into points of themerged OTDR trace proximate the first stitching point, and whereinstitching the portion of the second electrical signal to the portion ofthe third electrical signal further comprises mixing a second ratio ofthe second electrical signal and the third electrical signal trace intopoints of the merged OTDR trace proximate the second stitching point.